This invention relates to a digital radio receiver for receiving an electric wave from an artificial satellite (that may be called a “satellite wave”) or an electric wave on the ground (that may be called a “terrestrial wave”) to listen in a digital radio broadcasting and, in particular, to a patch antenna for use in the digital radio receiver.
In recent years, a digital radio receiver, which receives the satellite wave or the terrestrial wave to listen the digital radio broadcasting, has been developed and is put to practical use in the United State of America. The digital radio receiver is mounted on a mobile station such as an automobile and can receive an electric wave having a frequency of about 2.3 gigahertz (GHz) to listen in a radio broadcasting. That is, the digital radio receiver is a radio receiver which can listen in a mobile broadcasting. Inasmuch as the received wave has the frequency of about 2.3 GHz, a reception wavelength (resonance frequency) λ thereof is equal to about 128.3 mm. In addition, the terrestrial wave is an electric wave in which a signal where the satellite wave is received in an earth station is frequency shifted a little.
Inasmuch as the electric wave having the frequency of about 2.3 GHz is used in the digital radio broadcasting in the manner which is described above, it is necessary to set up an antenna outside the automobile. Accordingly, the antenna must be attached to a roof of the automobile in a case where the digital radio receiver is mounted in the automobile.
It is necessary for the antennas of the type described to have a wide directivity. This is because it is necessary for the digital radio receiver to receive, from the artificial satellite, the satellite wave having a relatively low angle of elevation which laid in a range of 20 degrees and 60 degrees. In addition, the terrestrial wave has an angle of elevation which is substantially equal to zero degree.
In general, it is understood that antennas of planer-type (plane-type) such as patch antennas are unsuited for the antenna for use in the digital radio receiver because the antennas of planer-type (plane-type) has a narrow directivity. However, even a patch antenna, it is known that it is possible for the patch antenna to widen a directivity of an antenna by using a ground (earth) plate having a wide area (see, e.g. JP 2003-163521 A).
Particularly, in a case where the digital radio receiver is mounted in the automobile, its antenna is attached to the roof of the automobile in the manner which is described above. In this event, inasmuch as the roof of the automobile serves as the ground plate in itself, it is confirmed that it is possible for the patch antenna to sufficiently use the antenna for the digital radio receiver.
On the other hand, there is a demand to listen in the digital radio broadcasting not only in the inside of the automobile but also, for example, from a portable audio apparatus such as a compact disc (CD) radio-cassette recorder. In order to reply the demand, it is proposed to mount the patch antenna in a lid which is arranged on a top surface of a casing in the portable audio apparatus and which is openable for taking an optical disc into and out from the portable audio apparatus. In this event, it is necessary to provide with the ground plate (earth plate) under the patch antenna.
In the manner which is described above, it is necessary for the patch antenna to use the ground plate having a large area in order to gain the directivity of the antenna in the low angle of elevation. However, when only the ground plate is provided, it is difficult to gain the directivity of the antenna in the low angle of elevation more. This is because the directivity of the patch antenna is uniquely determined by a size of the ground plate.
In addition, there is a microstrip antenna as one of the antennas of the planer-type. As the microstrip antenna, those where non-feeding elements are mounted at side edge portions of a top surface of an antenna substrate is known (see, e.g. JP 11-284429A). The non-feeding elements are mounted on the top surface of the antenna substrate by reason of inhibiting diffraction waves from an end part of a ground conductor. It is possible to improve an antenna characteristic in the microstrip antenna described in JP 11-284429 A. However, structure where the non-feeding elements are mounted on the top surface has large interference for a radiation element. In addition, inasmuch as it is impossible to adjust the non-feeding elements up and down, it is happened in a case where it is impossible to satisfy a desired antenna characteristic.
In addition, a compact plane patch antenna for use in a global positioning system (GPS) or the like is known (see, e.g. JP 07-094934 A). According to JP 07-094934 A, the compact plane patch antenna has high infrequency temperature characteristics and high reliability by using magnesium titanate ceramic having comparatively high dielectric constant as a main material for a dielectric material and adding the proper quantity of lithium niobate, alumina, manganese oxide, etc., individually or their combination at ions to the main material to mold the antenna.